The invention is directed to metal alloys for use in dentistry, especially for use with dental porcelains in making dental restorations.
The customary dental alloys having a gold content between 70 and 80 weight percent are well suited for the production of dental prostheses (e.g., German OS No. 1533233). They are resistant to corrosion in the oral cavity and can be worked with easily.
In recent times because of the high price of gold, alloys have been employed in which part of the gold content is replaced by palladium. In order to match the good melting casting properties of the high gold content alloys, the silver content has been increased in several of these reduced gold alloys (e.g., German Pat. No. 2440425). However, an aesthetic disadvantage of such alloys is that the dental porcelain can be discolored by the silver during the firing process.
The exact mechanism which causes such discoloration is not known. However, alloys without silver do not show this behavior.
The known silver free gold-palladium alloys (e.g., German Pat. No. 2813813 and related Knosp U.S. Pat. No. 4,179,286) in contrast have a high liquidus temperature, which makes melting difficult in the customary dental casting machines. Besides, such alloys do not show the mold-filling capacity and the flow properties of the high gold containing or the reduced gold, but silver containing alloys, which can lead to defects in the castings and to problems such as bubbles in the fired on porcelain. Alloys suited for firing on of dental porcelains must additionally posses a thermal expansion coefficient higher than that of the porcelain, in order to avoid tensile stresses in the porcelain layer which can lead to cracks in the porcelain. However, the expansion coefficient of the reduced gold, silver free alloys is found to be too low. This as well frequently leads to cracks or fissures in the porcelain.
In order to improve the industrial efficiency, it is necessary to reuse the casting waste. Therefore, the alloys must be melted several times and be able to cast without significantly changing their properties which can lead to hot tears in the castings or to spalling of the porcelain.
From Prasad U.S. Pat. No. 4,419,325, there are known palladium alloys for dental purpose which contain in addition to 35 to 85% palladium, 5 to 15% gallium, 0.1 to 0.5% ruthenium or rhenium, 0 to 50% gold, 0 to 12% copper, 0 to 5% aluminum, and 0 to 13% cobalt. These alloys have a relatively high melting range and do not show optimum bonding to the generally available porcelains. Besides they must be melted under vacuum or inert gas.
In German OS No. 2944755 there are described numerous dental porcelain alloys having about 32 to 63% gold and 29 to 58% palladium which in addition contain 0.5 to 10.55% indium and to which there can be added up to several percent of gallium, tin, cooper, nickel, aluminum, titanium, and silver. However, these alloys degrade with multiple remeltings and then no longer result in dense castings.
After firing the porcelain it is customary to braze together the different pieces of a dental bridge in a furnace. For this purpose, a flux is used which does not attack the ceramic. It is not possible with the alloys according to German OS No. 2944775 to get ductilely reliable joints with cadmium free brazes. The joint fractures without plastic deformation along the surface between the filler metal and the parent metal.
Hence it was the purpose of the present invention to provide noble metal alloys for dental purposes with reduced gold content compared to conventional gold alloys, especially for firing on of dental porcelains, which are low melting, have a high solidus temperature in order that they do not deform during the firing on of porcelain, do not lead to discolorations in the ceramic, make strong joints with cadmium free dental blazes. Besides, they should be repeatedly meltable and castable in air without degrading in any way.